Vacuum relief assembly

ABSTRACT

A vacuum relief assembly for relieving the vacuum created in a freezer chamber of an ultra low temperature freezer. The assembly comprises tubing having an inlet port open to the external environment and an exit port open to the freezer chamber, the inlet and exit ports in fluid communication with each other. A valve is coupled to the tubing that either permits or prevents fluid flow between the external environment and the freezer chamber. A heating element contacts the tubing and is operable to apply heat thereto so as to prevent the tubing from becoming occluded with condensation. The vacuum relief assembly may further include an activation switch for energizing the heating element and a timing circuit for turning the heating element off after a predetermined period of time.

This claims the benefit of U.S. Provisional Application Ser. No. 60/538,813, filed Jan. 23, 2004 and herein incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention generally relates to ultra low temperature upright freezers which are particularly useful in laboratories and other scientific environments.

BACKGROUND OF THE INVENTION

Several types of upright, ultra low temperature freezers are available, for example, for cooling various scientific products to very low temperatures. One such upright freezer has been available since prior to this invention from Thermo Electron Corp., located in Marietta, Ohio, under its “8600” and “900” series of freezers. These upright freezers use various controls but each traditionally include a single inner chamber cooled by a surrounding “cold wall” type refrigeration system operated by an ultra low temperature refrigeration unit disposed in the base of the freezer. Evaporation or cooling coils run up and down the side walls and back wall of the freezer as well as through the top and bottom walls. No fans are used for cooling the freezer chamber. Cooling is only accomplished by conduction and convection from the freezer walls into the chamber. A single full length upright door is mounted by hinges in a conventional fashion to the front of the freezer to provide access to the chamber. This particular freezer is capable of cooling the chamber to temperatures within the range of about −50 degrees Celsius to −86 degrees Celsius.

A problem which has arisen with such ultra low temperature freezers is that when the front door is opened, the extremely cold and heavy air within the chamber tends to spill out of the bottom of the chamber through the front opening of the freezer. Often, these freezers are used for containing items which must be maintained at a low temperature and which must be accessible in a repeated fashion throughout the day. The extremely cold and relatively heavy air sinks to the bottom of the freezer and spills or falls out the bottom of the front door opening resulting in a substantial loss of cold air every time the door is opened. The cold air that spills from the chamber is replaced by warm, moist ambient air. Due to the low temperatures maintained within the chamber, when the front door is closed, the moisture in the air quickly condenses to a solid, usually in the form of a frosting layer along the cold walls of the chamber. Moreover, removing the moisture from the air causes the pressure within the chamber to decrease creating a partial vacuum within the chamber. This pressure differential between the chamber and the ambient makes it difficult to reopen the freezer door without the application of excessive force.

Several previous ultra low temperature freezer designs provide for vacuum relief by including a vacuum relief port that draws air into the chamber when a pressure difference develops between the chamber and the ambient. The vacuum relief port generally includes some type of valve mechanism that opens when a vacuum is created within the chamber. Previous vacuum relief ports, however, have some drawbacks, such as becoming blocked with solid condensate and therefore requiring regular maintenance for proper operation. To this end, as the ambient air is pulled through the vacuum relief port, the moisture in the air quickly condenses and forms frost or ice within the relief port. Over a relatively short period of time, sometimes less than a day, depending on the usage of the ultra low temperature freezer, the frost or ice will accumulate so as to substantially occlude the port thereby preventing the relief of the vacuum within the chamber. To prevent the frost from occluding the port, the port must be cleared of any frost or ice on a regular basis.

It would therefore be desirable to provide an ultra low temperature freezer which allows repeated access to items contained therein without the application of excessive force and without the regular maintenance required for previous vacuum relief ports.

SUMMARY OF THE INVENTION

The present invention provides an improved ultra low temperature freezer that includes a vacuum relief assembly that relieves a vacuum created within the freezer chamber upon opening and closing the freezer door. An ultra low temperature freezer has a freezer chamber surrounded by an external environment and contained by insulated peripheral walls including a pair of side walls, a top wall, a bottom wall, a rear wall, and an insulated door hingedly secured to one of the side walls. The vacuum relief assembly comprises tubing having an inlet port open to the external environment and an outlet port open to the freezer chamber. The inlet port and exit port are in fluid communication with each other. The vacuum relief assembly further includes a first valve coupled to the tubing having an open and closed position. In the open position, the external environment and the freezer chamber are in fluid communication and fluid may flow through the valve. In the closed position, fluid is prevented from flowing through the valve. The vacuum relief assembly further includes a heating element coupled to the tubing and operable to apply heat thereto so as to prevent condensation from occluding the tubing.

The vacuum relief assembly may further include an activation switch operatively coupled to the heating element and operable to cause the heating element to produce heat. The activation switch may be further operatively coupled to freezer operations, such as opening or closing the freezer door or moving a latching handle on the freezer door. Moreover, the vacuum relief assembly may further include a timing circuit operatively coupled to the heating element and operable to cause the heating element to stop producing heat after a predetermined period of time. Alternatively, or in addition to the timing circuit, the vacuum relief assembly may include a thermal switch operatively coupled to the heating element and operable to cause the heating element to stop producing heat when at least a portion of the tubing reaches a specified temperature.

In one embodiment, the vacuum relief assembly is carried by the freezer door and includes a latching handle mounted to the freezer door and moveable between a first and second position. When the latching handle is in the first position, fluid is prevented from flowing between the external environment and the freezer chamber. When the latching handle is in the second position, fluid is permitted to flow between the external environment and the freezer chamber. The activation switch causes the heating element to produce heat when the latching handle is placed in the second position.

To relieve a vacuum in the freezer chamber, a user moves the latching handle from the first position to the second position thereby exposing the inlet port to ambient pressure. If a vacuum exists within the freezer chamber, fluid flows through the inlet port, through the exit port and into the freezer chamber. This equalizes the pressure between the freezer chamber and the external environment such that the freezer door may be opened without excessive force. Moreover, moving the latching handle to the second position energizes the heating element. The application of heat removes any condensation that may occlude the tubing such that when the freezer door is subsequently reopened, the tubing is free of condensation and vacuum relief may occur. A user then closes the freezer door and moves the latching handle from the second position to the first position, thereby preventing fluid communication between the external environment and the freezer chamber.

In another embodiment of the invention, the vacuum relief assembly includes tubing comprising a housing having a first and second end. The first end of the housing is configured as the inlet port open to the external environment. The tubing further includes a porting tube having a first end coupled to the second end of the housing. The second end of the porting tube is configured as the exit port open to the freezer chamber. A first valve, configured as a check valve, is coupled to the housing and has a open and closed position. When a vacuum is created within the freezer chamber, the check valve opens to allow fluid to flow into the chamber and relieve the vacuum. A heater, configured as a positive temperature coefficient heater is thermally coupled to the porting tube and operable to apply heat thereto so as to prevent condensation from occluding the tubing.

By virtue of the foregoing, an ultra low temperature freezer includes a vacuum relief assembly to relieve a vacuum often created upon opening and closing the freezer door. These and other objects and advantages of the invention shall be made apparent from the accompanying drawings and the description thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the general description of the invention given above and the detailed description of the embodiments given below, serve to explain the present invention.

FIG. 1 is a perspective view of an ultra low temperature freezer having a vacuum relief assembly of the present invention;

FIG. 2 is a perspective view of an exemplary embodiment of the vacuum relief assembly of the present invention;

FIG. 3 is a perspective view of an embodiment of the vacuum relief assembly incorporated into the door of an ultra low temperature freezer;

FIG. 4 is a disassembled perspective view of the latching handle and mounting pin that function as the valve for the vacuum relief assembly of FIG. 3, the valve being in the closed position;

FIG. 5 is an assembled perspective view of the latching handle and mounting pin that function as the valve for the vacuum relief assembly of FIG. 3, the valve being in the open position;

FIGS. 6-8 are top plan sequential views of the vacuum relief assembly of FIG. 3 showing the vacuum being relieved;

FIG. 9 is an enlarged top plan view of an alternate embodiment of a vacuum relief assembly of the invention incorporated into the door of an ultra low temperature freezer.

DETAILED DESCRIPTION OF THE DRAWINGS

With reference to FIG. 1, there is shown an exemplary embodiment of an upright ultra low temperature freezer 10 incorporating a vacuum relief assembly in accordance with the invention. The ultra low temperature freezer 10 generally comprises a freezer chamber 12 contained by an insulated top wall 14, an insulated bottom wall 16, a pair of insulated side walls 18, 20 and an insulated rear wall 22. An insulated freezer door 24 is hingedly mounted to side wall 18 and seals against front edges of top wall 14, bottom wall 16, and side walls 18, 20. Freezer door 24 may have a single panel construction, as shown in FIG. 1, or a multi-panel construction as is known in the art. The ultra low temperature freezer 10 may further include a latching handle 26 for securing freezer door 24 in the closed position and to facilitate opening freezer door 24 so as to gain access to freezer chamber 12.

In reference to FIG. 2, there is shown an exemplary embodiment of the vacuum relief assembly 28 of the invention. The vacuum relief assembly 28 comprises an inlet port 30 open or exposed to an external environment 32, an exit port 34 open or exposed to freezer chamber 12 and tubing 36 providing a fluid communication path between the inlet port 30 and the exit port 34. Tubing 36 may for example be copper tubing, plastic tubing, or other suitable structural materials known in the art. A valve 37 is coupled to tubing 36 between the external environment 32 and the freezer chamber 12 and has an open and closed position. Valve 37 permits fluid communication between external environment 32 and freezer chamber 12 when in the open position and prevents fluid communication when in the closed position. When the pressure at the inlet port 30 is greater than the pressure at the outlet port 34 and valve 37 is open, fluid may flow from the inlet port 30 to the outlet port 34. Valve 37 may be disposed in the tubing 36 between the inlet port 30 and the exit port 34 or may be disposed between the external environment 32 and the inlet port 30 such that fluid must travel through valve 37 to enter inlet port 30.

As shown in the exemplary embodiment of FIG. 2, the vacuum relief assembly 28 further includes a heater 38 coupled to a power source 35 and further coupled to tubing 36 between inlet port 30 and exit port 34. Heater 38 is operable to produce and apply heat to tubing 36 to defrost exit port 34 and/or a portion of tubing 36 adjacent exit port 34 from any frost or other condensation that may occlude the fluid communication path between the external environment 32 and the freezer chamber 12. Heater 38 may, for example, be a resistance type heater such as an electrical foil heater that can be wrapped around the exterior of tubing 36 along a portion thereof. Because the exit port 34 is exposed to the freezer chamber 12, heater 38 may be advantageously positioned adjacent exit port 34. Those having skill in the art will recognize other types of heaters that can be used in the present invention.

Because a goal of the ultra low temperature freezer 10 is to keep the contents (not shown) of the freezer chamber 12 at a low temperature, it can be counter productive and potentially harmful to the freezer contents to provide a heat source, such as heater 38, near the freezer chamber 12. In order to minimize the effects of heater 38, or other heat source, on freezer operation and freezer contents, the vacuum relief assembly 28 may further include an activation switch 40 that energizes heater 38 and causes heater 38 to produce heat along at least a portion of tubing 36. In this way, heater 38 does not operate in a continuous manner, so as to, for example, keep exit port 34 at a constant temperature above the dew point temperature, but may be operated in a selective manner so that heater 38 may be energized on the happening of a certain event, such as for example the opening or closing of door 24. The activation switch 40 may be a manual type of switch wherein a user positions the switch in a first position that activates heater 38. Alternatively, activation switch 40 may be tied to a user's normal operation of the freezer. For example, activation switch 40 may be a magnetic switch that detects the opening or closing of freezer door 24. In this way, when door 24 is placed in the open position after having been in the closed position, the activation switch 40 energizes heater 38 and causes heater 38 to produce heat. Activation switch 40 may also activate heater 38 by placing door 24 in the closed position after having been in the open position.

To keep heater 38 from continuously operating after having been energized by activation switch 40, vacuum relief assembly 28 may further include a timing circuit 42 that causes heater 38 to stop producing heat after a predetermined period of time. Thus after having been energized by activation switch 40, such as by opening the freezer door 24, timing circuit 42 provides power to heater 38 so as to produce and apply heat to tubing 36 to defrost the exit port 34 or a portion of tubing 36. After the predetermined period of time, timing circuit 42 cuts power to heater 38 so that heater 38 stops producing heat. In this way, the exit port 34 and tubing 36 may be cleared of any frost or other condensation so that a vacuum in freezer chamber 12 may be relieved without significantly affecting freezer operation or harming freezer contents. The amount of time required to clear the exit port 34 and/or tubing 36 depends on several factors, including the temperature inside the freezer chamber 12 and the amount of frost or condensation formed in the exit port 34 or tubing 36. It is contemplated that under most circumstances, energizing heater 38 for between approximately three to five minutes will clear the condensation without adversely affecting the freezer contents. The invention, however, is not so limited and timing circuit 42 may provide for adjusting the amount of time heater 38 remains energized.

Alternatively, to prevent heater 38 from operating in a continuous manner, vacuum relief assembly 28 may further include a thermal switch 44, such as for example a thermostat, in conjunction with heater 38. Thermal switch 44 may be adapted to monitor the temperature of the exit port 34 or a portion of tubing 36 and cause heater 38 to stop producing heat when the exit port 34 or portion of tubing 36 reaches a specified temperature. Thus after having been energized by activation switch 40, such as by opening the freezer door 24, the thermal switch 44 provides power to heater 38 so as to produce heat and apply it to tubing 36 to defrost the exit port 34 or a portion of tubing 36. When the exit port 34 or portion of tubing 36 reaches the specified temperature, thermal switch 44 cuts power to heater 38 so that heater 38 stops producing heat. The exit port 34 and tubing 36 may be cleared of any condensation so that a vacuum in freezer chamber 12 may be relieved without significantly affecting freezer operation or harming freezer contents. The specified temperature at which heater 38 stops producing heat may be set at, for example, 10° C. The invention, however, is not so limited and thermal switch 44 may provide for adjusting the temperature at which the heater 38 stops producing heat. It should also be realized that timing circuit 42 and thermal switch 44 may be used in combination with each other to prevent the exit port 34 or portion of tubing 36 from reaching, for example, a maximum specified temperature during the predetermined period of time. In this way, thermal switch 44 acts as a safety switch in the event timing circuit 42 fails or power is not cut to heater 38.

In further reference to FIG. 2, the exemplary embodiment of the vacuum relief assembly 28 may further include drain assembly 46 coupled to tubing 36 by another length of tubing 45 and adapted to collect condensation from tubing 36 and/or exit port 34. Tubing 36, 45 and drain assembly 46 are configured such that gravity drives the condensation through tubing 36, through tubing 45 and into drain assembly 46. This may be accomplished, for example, by angling tubing 36 adjacent where drain assembly 46 couples to tubing 36 so that condensation from exit port 34 or tubing 36 adjacent exit port 34 flows back through tubing 36 and into drain assembly 46 without flowing toward inlet port 30. Advantageously, drain assembly 46 may be formed from a gas permeable material, such as nylon or silicon, that allows water vapor to escape to the external environment 32 while retaining liquid water within drain assembly 46. Drain assembly 46 may further include a drain valve 47 at the bottom end of drain assembly 46 that is normally in a closed position but may be periodically opened so as to drain any water collected in drain assembly 46.

In continued reference to FIG. 2, the exemplary embodiment of vacuum relief assembly 28 may further comprise a second valve 49 located downstream of valve 37 and configured to act as a “slow leak” relief or bleed line for a vacuum in freezer chamber 12. If a vacuum is created in freezer chamber 12 after valve 37 is closed, freezer chamber 12 and portions of the valve relief assembly 28 downstream of valve 37 are under the vacuum. In the normal sense, air from the external environment 32 would slowly leak through the seals between the freezer door 24 and freezer walls 14, 16, 18, 20 so as to relieve the vacuum over a period of time. This might take, for example, on the order of approximately one hour. To reduce the time over which a vacuum within freezer chamber 12 might be relieved, without opening valve 37, second valve 49 is provided. Second valve 49 is configured to provide a small leak so that a vacuum within freezer chamber 12 may be relieved over a period of time shorter than the time to relieve the vacuum by a leak along the seal of the freezer door 24. Second valve 49 may, for example, be configured to relieve a vacuum in freezer chamber 12 in approximately thirty minutes.

As shown in FIG. 3, the vacuum relief assembly 28 of FIG. 2 may be readily incorporated into and carried by door 24 of an ultra low temperature freezer 10. The inlet port 30 is positioned in the exterior wall 48 of door 24 so that inlet port 30 may be open to the external environment 32. The outlet port 34 is positioned in the interior wall 50 of freezer door 24 so that when door 24 is closed, outlet port 34 is open to freezer chamber 12. As shown in FIGS. 4 and 5, adjacent the inlet port 30 is a mounting pin 52 for attaching latching handle 26 to freezer door 24. Latching handle 26 is configured to be moveable between a first and second position. In the first position, door 24 is closed and secured to walls 14, 16, 18, 20 of freezer 10. In the second position, door 24 may be opened by pulling latching handle 26 in an outward direction. Mounting pin 52 and latching handle 26 cooperate to function as valve 37 and may further cooperate to function as valve 49, as will now be explained.

When latching handle 26 is in the first position, as shown in FIG. 4, the external environment 32 and inlet port 30 are not in fluid communication with each other. When the latching handle 26 is in the second position as, shown in FIG. 5, the external environment 32 and inlet port 30 are in fluid communication with each other. As shown in FIGS. 4 and 5, this could be done, for example, by having an access port 54 in mounting pin 52 and an access port 56 in latching handle 26 such that access ports 54, 56 are aligned when latching handle 26 is in the second position and valve 37 is open. Otherwise, access ports 54, 56 are not aligned and thus fluidly sealed from each other so that valve 37 is closed. Furthermore, mounting pin 52 and latching handle 26 may be configured to provide a small “leak” so as to relieve a vacuum in freezer chamber 12 over a period of time, as previously discussed. This may be done, for example, by having a small leakage port 57 in mounting pin 52 such that when latching handle 26 is in the first position leakage port 57 is aligned with access port 56 and thus open to external environment 32. Leakage port 57 has a small diameter, such as between approximately 20-30 thousandths of an inch. In this way, a vacuum within freezer chamber 12 can be slowly relieved through leakage port 57 over a period of time, such as, for example, approximately 30 minutes.

In addition, latching handle 26 may activate heater 38. Activation switch 40 may be configured such that when freezer door 24 is to be opened and latching handle 26 is moved to the second position, activation switch 40 energizes heater 38 and causes heater 38 to produce heat. It should be recognized, however, that activation switch 40 can activate heater 38 by placing latching handle 26 in the first position, such as when closing the freezer door 24.

As shown in FIGS. 6-8, the vacuum relief assembly 28, as shown in FIG. 3, can be used to relieve a vacuum created within a freezer chamber 12 of an ultra low temperature freezer 10. In FIG. 6, the freezer door 24 is in the closed position, latching handle 26 is in the first position and therefore there is no fluid communication between the external environment 32 and the freezer chamber 12. Depending on the time freezer door 24 was last closed and the amount of moist air in freezer chamber 12 when freezer door 24 was last closed, a vacuum may exist in freezer chamber 12. A user (not shown) moves, by for example rotating, the latching handle 26 from the first position to the second position to facilitate the opening of freezer door 24. As shown in FIG. 7, movement of the latching handle 26 to the second position provides a fluid communication path between the external environment 32 and inlet port 30. Inlet port 30 is then exposed to ambient pressure and if a vacuum exists, fluid flows through inlet port 30, through outlet port 34 and into freezer chamber 12. This movement of fluid equalizes the pressure between freezer chamber 12 and the external environment 32 such that freezer door 24 may be opened without excessive force to overcome the vacuum, as shown in FIG. 8. Additionally, movement of latching handle 26 to the first position energizes heater 38 to produce heat. Condensation from the heating process drains into drain assembly 46. Heater 38 produces heat for the predetermined period of time. Timing circuit 42 then cuts power to heater 38 and heater 38 stops producing heat. In this way, when the freezer door 24 is subsequently reopened, the exit port 34 and tubing 36 are free of condensate and vacuum relief may occur. A user then closes door 24 and moves latching handle 26 from the second position to the first position to secure door 24 in the closed position. Movement of latching handle 26 to the first position closes the fluid communication path between the external environment 32 and inlet port 30 through valve 37. If a vacuum is created in freezer chamber 12 after the latching handle 26 is moved to the first position and the freezer door 24 has not been opened over an extended period of time, a vacuum may be relieved by the slow leak through leakage port 57 of second valve 49.

In FIG. 9, in which like reference numerals refer to like features in FIGS. 1 -8, an alternate embodiment of a vacuum relief assembly 60 in accordance with the invention comprises an inlet port 62 open or exposed to the external environment 32, an exit port 64 open or exposed to freezer chamber 12, and tubing 66 providing a fluid communication path between the inlet port 62 and exit port 64. Tubing 66 includes a housing 68 having inlet port 62 at one end thereof and a porting tube 70 coupled to the other end of the housing 68 and having exit port 64. The housing 68 may, for example, be made of a suitable engineering plastic through a machining or molding process and the porting tube 70 may be made of brass or other suitable materials known in the art. The vacuum relief assembly 60 further includes a spring-loaded check valve 72 located within the housing 68 and forming a portion of the fluid communication path between the inlet port 62 and exit port 64.

Check valves are well known in the art and may, for example, be commercially obtained through Neoperl Inc. located in Waterbury, Conn. Check valve 72 has an open and closed position and permits fluid communication between the external environment 32 and freezer chamber 12 when in the open position and prevents fluid communication when in the closed position. When the pressure at the inlet port 62 is greater than the pressure at the exit port 64, such as when a vacuum is created within freezer chamber 12, the check valve automatically opens and fluid may flow from the inlet port 62 to exit port 64. As the pressure between the freezer chamber 12 and external environment 32 equalizes, the check valve 72 moves toward the closed position to seal the fluid communication path.

As shown in the embodiment in FIG. 9, the vacuum relief assembly 60 further includes a heater 74 coupled to a power source 35 and further coupled to tubing 66 adjacent exit port 64, such as along porting tube 70. As in the previous embodiment, the heater 74 is operable to produce and apply heat to tubing 66 to defrost exit port 64 and/or a portion of porting tube 70 adjacent exit port 64 from any frost or other condensation that may occlude the fluid communication path between the external environment 32 and the freezer chamber 12. Heater 74 may be a resistance type heater such as a positive temperature coefficient (PTC) thermistor heater that can be wrapped around the exterior of porting tube 70 and along a portion thereof. As will be explained in more detail below, the PTC heater provides some advantages over conventional resistance heaters.

The operation of the ultra low temperature freezer 10 having the vacuum relief assembly 60 incorporated therein works in a manner similar to that shown and described above. In particular, vacuum relief assembly 60 includes an activation switch 40 that energizes heater 74 and causes heater 74 to produce heat along at least a portion of porting tube 70. Activation switch 40 may be a manual type of switch, but is preferably tied to the normal operation of the freezer, such as by opening or closing the freezer door 24. In this way, when door 24 is placed in the open position after having been in the closed position, the activation switch energizes heater 74 and causes heater 74 to produce heat. Activation switch 40 may alternately activate heater 74 by placing door 24 in the closed position after having been in the open position.

Vacuum relief assembly 60 may further include a timing circuit 42 that causes heater 74 to stop producing heat after a predetermined period of time. Thus, after having been energized by activation switch 40, such as by opening freezer door 24, timing circuit 42 provides power to heater 74 so as to produce and apply heat to porting tube 70 to defrost the exit port 64 or a portion of porting tube 70. After the predetermined period of time, timing circuit 42 cuts power to heater 74 so that heater 74 stops producing heat. In this way, the exit port 64 may be cleared of any frost or other condensation so that a vacuum in freezer chamber 12 may be relieved without significantly affecting freezer operation or harming freezer contents. As with the previous embodiment, it is contemplated that under most circumstances, energizing heater 74 for between approximately three to five minutes will clear the condensation without adversely affecting the freezer contents. The invention, however, is not so limited as timing circuit 42 may provide for adjusting the amount of time heater 74 remains energized.

The heater 74 is preferably a PTC thermistor heater. This type of heater is inherently self-regulating so as to prevent an overheating or a run away heating condition that would adversely affect the contents in freezer chamber 12. To this end, the PTC heater advantageously contains a ceramic heating element having an electrical resistance that depends on the temperature of the ceramic heating element. This characteristic allows the PTC heater to be effectively self-regulating so as to operate at an equilibrium temperature, called the switch temperature, as will now be explained. When a constant voltage source is applied across the heating element and the heating element is, for example, at its equilibrium point, the resistance of the heating element and the current flowing through the heater are such that the generated resistance heating, defined by I²R, is equal to the heat loss from the heating element, thus maintaining equilibrium. If the heat loss from the heating element decreases, the temperature of the heating element increases. As the temperature of the heating element increases, so does its electrical resistance causing the current flowing through the heating element to decrease. This in turn causes the generated resistance heating to decrease, thus decreasing the temperature of the heating element toward the switch temperature.

Conversely, if the heat loss from the heating element increases, the temperature of the heating element decreases. As the temperature of the heating element decreases, so does its electrical resistance causing the current flowing through the heating element to increase. This in turn causes the generated resistance heating to increase, thus increasing the temperature of the heating element toward the switch temperature. In this way, the PTC heater is thus self regulating so as to have a temperature approximately equal to its switch temperature. Moreover, by varying the materials of the ceramic heating element, the switch temperature may be selectively chosen for a particular application. For example, a PTC heater having a switch temperature of approximately 40° C. may be used in the invention and is commercially available through DBK, Inc. located in Cincinnati, Ohio. PCT heaters are advantageous because the self-regulating feature allows for the elimination of a thermal switch 44, such as a thermostat, or other circuitry designed to cut power to heater 72 in the event activation switch 40 fails.

While the present invention has been illustrated by the description of embodiments thereof, and while the embodiments have been described in considerable detail, it is not intended to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is, therefore, not limited to the specific details, representative apparatus and method, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of the general inventive concept. 

1. A vacuum relief assembly for an ultra low temperature freezer, the freezer having a freezer chamber surrounded by an external environment and contained by insulated peripheral walls including a pair of side walls, a top wall, a bottom wall, and a rear wall, and an insulated door hingedly secured to one of the side walls, said assembly comprising: tubing having an inlet port open to the external environment and an exit port open to the freezer chamber, said inlet port in fluid communication with said exit port; a first valve coupled to said tubing and having an open and closed position, said first valve permitting fluid communication between the external environment and the freezer chamber when in the open position, said first valve preventing fluid communication between the external environment and the freezer chamber when in the closed position; and a heating element contacting said tubing and operable to apply heat thereto so as to prevent condensation from occluding said tubing.
 2. The vacuum relief assembly of claim 1, wherein said heating element is a foil heating element wrapped around at least a portion of said tubing.
 3. The vacuum relief assembly of claim 1, further comprising: an activation switch operatively coupled to said heating element and operable to cause said heating element to produce heat.
 4. The vacuum relief assembly of claim 3, wherein the freezer door has an open and closed position, said activation switch further operatively coupled to the freezer door and operable to cause said heating element to produce heat when the freezer door is placed in at least one of the open and closed positions.
 5. The vacuum relief assembly of claim 3, wherein the freezer further comprises a latching handle mounted to the freezer door and being moveable between first and second positions, said activation switch further operatively coupled to the latching handle and operable to cause said heating element to produce heat when the latching handle is placed in at least one of the first and second positions.
 6. The vacuum relief assembly of claim 3, further comprising: a timing circuit operatively coupled to said heating element and operable to cause said heating element to stop producing heat after a predetermined period of time.
 7. The vacuum relief assembly of claim 3, further comprising: a thermal switch operatively coupled to said heating element and operable to cause said heating element to stop producing heat when at least a portion of said tubing reaches a specified temperature.
 8. The vacuum relief assembly of claim 1, wherein the freezer further comprises a latching handle mounted to the freezer door, the latching handle being moveable between first and second positions, said first valve being in the open position when the latching handle is placed in the second position, said first valve being in the closed position when the latching handle is placed in the first position.
 9. The vacuum relief assembly of claim 8, wherein said first valve further comprises: a mounting pin coupled to said inlet port and having an access port in fluid communication with said inlet port; said latching handle coupled to said mounting pin and further comprising an access port in fluid communication with the external environment, said access port in said mounting pin in fluid communication with said access port in said latching handle when said latching handle is in the second position, said access port in said mounting pin fluidly sealed from said access port in said latching handle when said latching handle is in the first position.
 10. The vacuum relief assembly of claim 1, wherein said assembly is carried by the freezer door.
 11. The vacuum relief assembly of claim 1, wherein said first valve is a check valve.
 12. The vacuum relief assembly of claim 1, wherein said heating element is a positive temperature coefficient heating element.
 13. An ultra low temperature freezer comprising: a freezer chamber surrounded by an external environment and contained by insulated peripheral walls including a pair of side walls, a top wall, a bottom wall, and a rear wall; an insulated freezer door hingedly secured to one of the side walls; and a vacuum relief assembly coupled to said freezer comprising: tubing having an inlet port open to the external environment and an exit port open to the freezer chamber, said inlet port in fluid communication with said exit port; a first valve coupled to said tubing and having an open and closed position, said first valve permitting fluid communication between the external environment and the freezer chamber when in an open position, said first valve preventing fluid communication between the external environment and the freezer chamber when in the closed position; and a heating element contacting said tubing and operable to apply heat thereto so as to prevent condensation from occluding said tubing.
 14. The ultra low temperature freezer of claim 13, wherein said heating element is a foil heating element wrapped around at least a portion of said tubing.
 15. The ultra low temperature freezer of claim 13, further comprising: an activation switch operatively coupled to said heating element and operable to cause said heating element to produce heat.
 16. The ultra low temperature freezer of claim 15, wherein said freezer door has an open and closed position, said activation switch further operatively coupled to said freezer door and operable to cause said heating element to produce heat when said freezer door is placed in one of the open and closed positions.
 17. The ultra low temperature freezer of claim 15, wherein said freezer further comprises a latching handle mounted to said freezer door and being moveable between first and second positions, said activation switch further operatively coupled to said latching handle and operable to cause said heating element to produce heat when said latching handle is placed in one of the first and second positions.
 18. The ultra low temperature freezer of claim 15, further comprising: a timing circuit operatively coupled to said heating element and operable to cause said heating element to stop producing heat after a predetermined period of time.
 19. The ultra low temperature freezer of claim 15, further comprising: a thermal switch operatively coupled to said heating element and operable to cause said heating element to stop producing heat when at least a portion of said tubing reaches a specified temperature.
 20. The ultra low temperature freezer of claim 13, wherein the freezer further comprises a latching handle mounted to the freezer door, the latching handle being moveable between first and second positions, said first valve being in the open position when the latching handle is placed in the second position, said first valve being in the closed position when the latching handle is placed in the first position.
 21. The ultra low temperature freezer of claim 20, wherein said first valve further comprises: a mounting pin coupled to said inlet port and having an access port in fluid communication with said inlet port; said latching handle coupled to said mounting pin and further comprising an access port in fluid communication with the external environment, said access port in said mounting pin in fluid communication with said access port in said latching handle when said latching handle is in the second position, said access port in said mounting pin fluidly sealed from said access port in said latching handle when said latching handle is in the first position.
 22. The ultra low temperature freezer of claim 13, wherein said assembly is carried by the freezer door.
 23. The ultra low temperature freezer of claim 13, wherein said first valve is a check valve.
 24. The ultra low temperature freezer of claim 13, wherein said heating element is a positive temperature coefficient heater.
 25. A vacuum relief assembly for an ultra low temperature freezer, the freezer having a freezer chamber surrounded by an external environment and contained by insulated peripheral walls including a pair of side walls, a top wall, a bottom wall, and a rear wall, and an insulated door hingedly secured to one of the side walls, said assembly comprising: tubing comprising a housing having a first and second end, said first end configured as an inlet port open to the external environment, and a porting tube having a first end coupled to said second end of said housing, said porting tube having a second end configured as an exit port open to the freezer chamber, said inlet port in fluid communication with said exit port; a first valve coupled to said tubing along said housing and having an open and closed position, said first valve permitting fluid communication between the external environment and the freezer chamber when in an open position, said first valve preventing fluid communication between the external environment and the freezer chamber when in the closed position; and a heater thermally coupled to said tubing along said porting tube and operable to apply heat thereto so as to prevent condensation from occluding said tubing.
 26. The vacuum relief assembly of claim 25, wherein said first valve is a check valve.
 27. The vacuum relief assembly of claim 25, wherein said heater is a positive temperature coefficient heater.
 28. A method for relieving a vacuum between a freezer chamber of an ultra low freezer and an external environment comprising: providing a fluid communication path between the external environment and the freezer chamber; opening a valve to permit fluid flow along the communication path so as to relieve the vacuum; and using a heater to heat at least a portion of the communication path between the external environment and the freezer chamber so as to prevent condensation from occluding the communication path.
 29. The method of claim 28, wherein the freezer includes a freezer door having an open and closed position, the method further comprising: causing the heater to produce heat when the freezer door is placed in at least one of the open and closed positions.
 30. The method of claim 29, further comprising: causing the heater to stop producing heat after a predetermined period of time.
 31. The method of claim 29, further comprising: causing the heater to stop producing heat when the temperature of at least a portion of the communication path reaches a specified temperature. 